Pull Request: Add Label Alignment & Touchbar Background Image Support

🎯 Summary

This PR introduces two key features:

1. Label Alignment Control - Users can now align key/dial labels to left, center, or right
2. Touchbar Background Images - Full support for custom background images on Stream Deck Plus touchbars

✨ Features

Label Alignment Property

Adds horizontal text alignment options for key labels, giving users more control over label positioning.

Changes:

• Added alignment field to KeyLabel dataclass with values: "left", "center", "right"
• New AlignmentButtons widget in the Label Editor with intuitive toggle buttons
• Text rendering now respects alignment when positioning labels
• Alignment is composable (page → action) and supports reset to default

UI:

Touchbar Background Image Handling

Enables users to set custom background images for the Stream Deck Plus touchbar/touchscreen.

Changes:

• New ImageRow in BackgroundEditor for touchscreen inputs
• Image selection via asset chooser with preview thumbnail
• Background images are resized to exact touchscreen dimensions
• Supports layering: background image → background color overlay → dial images
• Clear button to remove background image

📁 Files Changed

🔧 Technical Details

Label Alignment Logic:

• Alignment affects both text anchor point and x-position calculation
• Left alignment uses padding from left edge
• Right alignment uses padding from right edge
• Center alignment (default) centers text horizontally

Background Image Compositing Order:

1. Load background image (if set) → resize to touchscreen dimensions
2. Apply background color overlay (if alpha > 0)
3. Composite dial images on top

🧪 Testing

• Test label alignment on keys (left/center/right)
• Test label alignment on dials
• Test alignment persistence across page reloads
• Test alignment reset button functionality
• Test touchbar background image selection
• Test touchbar background image with color overlay
• Test touchbar background image clear functionality
• Test that ImageRow only appears for touchscreen inputs

📝 Notes

• Default alignment is "center" (backwards compatible)
• Alignment UI is hidden when label text is empty
• Background image support is exclusive to touchscreen inputs (Stream Deck Plus)